[EricH]

Adam, have you ever seen an R/C aircraft built mostly out of aluminum? (I have, but it was a custom build for SAE Aero Design--and that particular airplane was still about 1/2 balsa.)

My contention is that it's not about what material you use, it's how you use it. Build the robot out of steel, but mess up something, and it will break. You could also build out of mainly plastic, and have it never break (1714, for example), because you did it right.